This assumes water is inconsistent. Once the rain butt is full, there's no more benefit. And then in a long dry season once it's emptied, it's not saving any more.
> Fix a leaking toilet – leaky loos can waste 200-400 litres a day.
If you're on a water meter you're already incentivised to fix this - so a better answer is water meters.
> Use water from the kitchen to water your plants.
Assuming you're using waste water, which most people won't.
> Avoid watering your lawn – brown grass will grow back healthy.
Households with lawns are rarer than they used to be - a big red flag has always been that hosepipe bans never applies to golf courses which use a large amount of water to keep their grass green all year.
> Turn off the taps when brushing teeth or shaving.
A few minutes of water p/person per day.
> Take shorter showers.
Probably minimal and potentially a hygiene problem.
> Delete old emails and pictures as data centres require vast amounts of water to cool their systems.
Negligible at best.